By hvhipot 
Series resonant voltage withstand device (also known as series resonant) is a resonant device used in power preventive testing for AC voltage withstand of large capacity and high voltage test objects. It is a complete set of voltage withstand devices composed of variable frequency power supply, excitation transformer, reactor, compensation capacitor, and capacitive voltage divider. The principle is simple, but the process is complex. By changing the inductance and test frequency of the resonant system, the circuit is in a resonant state to achieve the purpose of the test. As the design of the capacity and voltage level of the voltage withstand device depends on the capacitance C of the test sample, the test voltage U, and the test frequency f, it is commonly used for power frequency withstand of power equipment such as power cables, transformers, GIS switches, etc. Below, I will introduce the role and significance of the components of the voltage withstand device in the entire series resonant device.
Variable frequency power supply: Firstly, SPWM (Pulse Width Modulation) control technology and IGBT circuit are used to produce pure sine waves in the series resonant circuit. The output frequency and voltage range can be adjusted to achieve manual, automatic, and semi-automatic testing control methods, monitoring the voltage, current, faults, etc. of the series and parallel resonant devices.
Excitation transformer: The design principle of excitation transformer is equivalent to that of power transformer, but in series and parallel resonance device systems, it mainly provides the stability of the complete equipment and improves the capacity of the test power supply.
Reactor: When a conductor is energized, a magnetic field is generated within a certain spatial range it occupies. Therefore, all conductors that can carry current have a general sense of inductance. However, the inductance of long straight conductors when energized is relatively small, and the magnetic field generated is not strong. Therefore, reactors are in the form of coils wound with wires. According to their capacity, reactors are divided into dry-type reactors and oil immersed reactors. Therefore, reactors mainly provide inductance. By adjusting the frequency (30-300Hz), the AC test voltage can be generated through capacitor resonance of the test sample. The test voltage and capacity can be adjusted by adjusting the number of parallel reactors.
Compensation capacitor: Compensation capacitor is an additional device used for capacitance compensation of large capacity resonance test devices and small capacity test objects. It can be configured according to the usage conditions considered during design. In practical applications, compensation capacitors can also be used to adjust the frequency in a small range.
Capacitive voltage divider: The capacitive voltage divider monitors and controls high voltage through the voltage division ratio. When the target voltage of the variable frequency power supply is set, the collected signal of the fully automatic control mainly comes from the voltage divider during testing, including the breakdown voltage value after breakdown flashover protection occurs.
Overall, the resonant withstand voltage device is a very practical series resonant testing device. Due to its wide frequency range and excellent performance, it can meet the power frequency withstand voltage of many test objects and is widely used in power, power generation, energy and other sectors.